The interaction of nitric oxide ( NO) with Rose Bengal (RB) in the presence of electron donors was investigated. Upon illumination of a mixture of RB and NO with visible light, an enhancement in the rate of NO consumption was observed that increased with increasing RB concentration. In the presence of electron donors (NADH, glutathione, or ascorbate), the rates of NO depletion increased further. NADH enhanced NO depletion to a greater extent than either glutathione or ascorbate. Photoactivated RB under anaerobic conditions reacts with NADH to form the RB anion radical (RB ), which has a characteristic visible absorption band centered at 418 nm. Rose Bengal anion radical disproportionates to give RB and a colorless reduced form of RB, RBH . The net result of this process is the photobleaching of RB. The presence of NO during irradiation of RB and NADH introduced a lag time into the kinetics of RB photobleaching. The length of this lag time was proportional to the concentration of NO. A similar lag time, which was also dependent on the NO concentration, was observed in the kinetics of formation of RB . The three-line electron spin resonance (ESR) spectrum of RB , with an intensity ratio of 1:2:1, was obtained during irradiation of RB and NADH under anaerobic conditions. NO introduced a concentration-dependent lag time into the kinetics of the appearance of this ESR signal. We postulate that NO oxidized RB to generate RB and, thus, inhibit photobleaching until NO is consumed. This reaction predicts the formation of NO , the one-electron reduced form of NO. Nitrous oxide, a characteristic dimerization product of NO , was detected by gas chromatography. This evidence indicates the occurrence of a type I mechanism between photoactivated RB and NO.